Flow measurement and control are an important means in the field of industry. A flow meter can be any of an assortment of devices meant to measure the speed of a mass as it passes through a closed space. A thermal flow meter is one kinds of the thermal flow meter. As a mass moves through a closed space, the thermal flow meter having sensors detects the flow rate by measuring the heat transferred from the surface of the sensors to the flowing mass. The sensor comprises a Wheatstone bridge having a upstream sensor coil Rcoil11, a downstream sensor coil Rcoil12, a resistor R11 and a resistor R22 usually, as shown in FIG. 1. Both the upstream sensor coil Rcoil11 and the downstream sensor coil Rcoil12 are heating elements. When the flowing mass passes by the upstream sensor coil Rcoil11 and the downstream sensor coil Rcoil12, the surface temperature of the upstream sensor coil Rcoil11 and the downstream sensor coil Rcoil12 change accordingly, this results in the change of their resistances which are reflected by their voltages. Then the flow rate of the mass can be obtained by measuring the voltage values.
However, the upstream sensor coil Rcoil11 and the downstream sensor coil Rcoil12 are unbalanced because of the manufacturing error in most cases. When the temperature changed, the temperature drift caused by the unbalance may be occurred. Generally, the degree of the sensor affected by temperature is determined by the quality of the sensor.
Among most products of the thermal sensors in the prior art, there is not a method is offered to compensate the temperature drift but to lower the tolerance and raise the precision of components. In this way, the ordinary effect is obtained together with huge waste. Moreover, the thermal sensors in the prior art generally work in the driving mode of constant current, and the sensors tend to be affected by temperature and environment, which brings about a larger null shift and a poor precision for products.